1. Field of the Invention
The present invention relates to a quartz crystal unit, and more particularly to a crystal unit having a high resonant frequency of 100 MHz or higher.
2. Description of the Related Art
Crystal units which have a quartz crystal blank housed in a casing are widely used as a frequency or time reference source for oscillators in various communication devices. In recent years, as the frequency band for use in communications is shifted to higher frequencies, crystal units are required to have a higher resonant frequency. To meet this requirement, there has been developed a crystal unit comprising a crystal blank which has a recess defined in a vibrating region thereof to reduce the thickness of the crystal blank at the recess for a higher resonant frequency, and which also has a relatively thick portion around the recess to hold the vibrating region for desired mechanical strength.
FIG. 1 shows such a conventional crystal unit in exploded perspective. As shown in FIG. 1, the conventional crystal unit comprises casing 1 made of ceramics or the like and having a recess defined therein, and quartz crystal blank 2 housed in casing 1. Casing 1 is substantially in the shape of a rectangular parallelepiped, and crystal blank 2 is of a substantially rectangular shape. Casing 1 has a step on one side of the recess, and a pair of connecting terminals 3 is disposed on opposite ends of an upper surface of the step for electrically connecting to crystal blank 2. Although not shown, a pair of mounting terminals for surface-mounting this crystal unit on a wiring board is formed on an outer surface of casing 1. The mounting terminals are electrically connected to connecting terminals 3 through via holes defined in casing 1.
Structural details of crystal blank 2 are shown in FIGS. 2A and 2B. Crystal blank 2 typically comprises an AT-cut quartz crystal blank. The AT-cut crystal blank has its resonant frequency determined by its thickness such that the thinner the crystal blank, the higher the resonant frequency. For crystal blank 2 to have a resonant frequency in excess of 100 MHz, crystal blank 2 has hole 4 defined centrally on one principal surface thereof to make the thickness of crystal blank 2 at the bottom of hole 4 smaller than a peripheral region around hole 4, the reduced-thickness region serving as a vibrating region. In the vibrating region, excitation electrodes 5 are formed respectively on both principal surfaces of crystal blank 2. From excitation electrodes 5, there extend respective extension electrodes 6 toward respective opposite ends of one shorter side of crystal blank 2. Extension electrodes 6 are associated with respective connecting terminals 3 on the step of casing 1. Extension electrode 6 disposed on the upper surface, as shown, of crystal blank 2 is folded back onto the lower surface, as shown, of crystal blank 2 at the above-described shorter side thereof. The opposite ends of the above-described shorter side of crystal blank 2 are fixed to respective connecting terminals 3 by joining members 7 such as of an electrically conductive adhesive, thereby holding crystal blank 2 horizontally in the recess in casing 1 and establishing electrical connection of connecting terminals 3 to extension electrodes 6, so that the mounting terminals on the outer surface of casing 1 are electrically connected to respective excitation electrodes 5 on crystal blank 2. The electrically conductive adhesive comprises an adhesive primarily composed of a relatively hard synthetic resin such as epoxy resin or the like.
After crystal blank 2 is fixed to the step in the recess, the opening of the recess is sealed by a cover (not sown), thus hermetically sealing crystal blank 2 in casing 1.
With the above conventional crystal unit, the laminated ceramics of casing 1 and crystal blank 2 have widely different thermal expansion coefficients. Specifically, the ceramics of casing 1 has a thermal expansion coefficient of about 7.0×10−6/° C., and crystal blank 2 has a thermal expansion coefficient in the range from 14.5 to 16.9×10−6/° C. Therefore, when the crystal unit is exposed to a high temperature environment, crystal blank 2 is strained due to the difference between the thermal expansion coefficients of casing 1 and crystal blank 2.
FIGS. 3A and 3B show how crystal blank 2 is strained due to the difference between the thermal expansion coefficients of casing 1 and crystal blank 2.
Since crystal blank 2 is held at the opposite ends of one shorter side thereof on casing 1 here, stresses are produced in the transverse direction of crystal blank 2 as shown in FIGS. 3A and 3B. Since crystal blank 2 has a larger thermal expansion coefficient, crystal blank 2 is subjected to compressive stresses. These applied compressive stresses concentrate on the thinner region of crystal blank 2, i.e., the vibrating region which is provided by hole 4 for a higher resonant frequency, resulting in a larger strain in the vibrating region. Usually, the frequency vs. temperature characteristics of the resonant frequency of the AT-cut quartz crystal blank is represented by a cubic curve having an inflection point near the normal temperature of 25° C. As the strain in the vibrating region becomes larger, the frequency vs. temperature characteristics becomes worse, making the oscillating frequency of a crystal oscillator which incorporates the crystal unit, unstable with respect to temperature changes. The strain in the vibrating region also adversely affects other vibrating characteristics of crystal blank 2.
When crystal blank 2 is fixed to the step in the recess in casing 1 by the electrically conductive adhesive, crystal blank 2 tends to be strained by a shrinkage of the electrically conductive adhesive at the time it is thermoset. This strain due to the shrinkage of the electrically conductive adhesive also adversely affects crystal blank 2.
The adverse effects posed by the stresses produced due to the difference in thermal expansion coefficients and the strain caused by a shrinkage of the electrically conductive adhesive manifest themselves as the vibrating region is thinner, i.e., as the resonant frequency of the crystal unit is higher. The adverse effects also manifest themselves as the crystal blank or the crystal unit is required to meet more stringent specifications.
To reduce stresses applied to the crystal blank, it has been practiced to use an electrically conductive adhesive primarily composed of a highly pliable synthetic resin such as silicone resin or the like for thereby allowing the crystal blank to have desired vibrating characteristics. However, the pliable electrically conductive adhesive tends to make the crystal unit less resistant to mechanical shocks that are applied thereto.